It is often necessary to operate different sections of an electronic circuit at different voltage levels. Such a necessity arises, for example, in printed circuit board testers in which the test points of the printed circuit board are switched to a high voltage bus by FET switches controlled by low voltage CMOS logic.
Likewise, in other systems, the need often arises to interconnect logic elements from different logic families, or to interconnect logic elements operated at different supply potentials. This need is met by the use of voltage level shifters, and a simplified voltage level shifter incorporating the concept of the invention used the switching circuit of the present invention to switch a printed circuit board test point, for example, to a voltage bus of the order of 100-500 volts, under the control of CMOS logic operating at a voltage level of from 0-10 volts.
As pointed out in U.S. Pat. No. 4,538,104, the General Electric Company introduced the first printed circuit board in-circuit tester over thirty (30) years ago. The in-circuit tester is designed to analyze the individual components on the printed circuit board. In this test, multiple probes are connected to a circuit analyzer at one end, and are all respectively connected to the test points of a printed circuit board at the other end. These probes are then switched individually to buses extending to the circuit analyzer, and which are known as the "TO" bus and the "FROM" bus. It is usual in present day test circuits for the "FROM" bus, for example, to be operated in a voltage range of from 100 to 500 volts, and for the "TO" bus to operate essentially at ground potential.
In actual test equipment, the switches used in the in-circuit testers are usually field effect transistors (FETS), and the FETS are operated by low voltage CMOS logic. There is no problem in switching the printed circuit board test points to the "TO" bus, because that bus is operated near ground potential, which is in the same voltage range as the voltage of the CMOS logic operating the particular FETS. However, problems arise in operating the FETS connecting the test points to the "FROM" bus, because, as mentioned, the "FROM" bus operates in a voltage range of 100-500 volts.
An objective of the present invention is to provide an improved and simplified voltage level shifting circuit which permits CMOS logic effectively to control the FET switches which connect the test point to the high voltage "FROM" bus.